1. Field of the Invention
The invention relates generally to an oil supply system for a vehicle, which includes a mechanical oil pump that is driven by an engine and an electric oil pump that is driven by an electric motor, and, more specifically to a technology for improving the rising characteristics of the hydraulic pressure produced by the mechanical oil pump during startup of an engine.
2. Description of the Related Art
Japanese Patent Application Publication No. JP-2005-315271 (JP-A-2005-315271) describes an oil supply system for a vehicle, which includes a mechanical oil pump that is driven by an engine and an electric oil pump that is driven by an electric motor. In the oil supply system, an oil discharge passage for the mechanical oil pump and an oil discharge passage for the electric oil pump are connected to each other, and the oil is supplied to a predetermined hydraulic pressure control circuit through these oil discharge passages. The oil supply system may be mounted in an eco-run vehicle in which an engine is stopped when the vehicle is stopped or a hybrid vehicle in which an engine and an electric motor are used in combination as drive power sources.
The mechanical oil pump may be at least partially above the oil level depending on the manner in which the mechanical oil pump is connected to the engine, the manner in which the mechanical oil pump is mounted in the vehicle, etc. Therefore, for example, while the engine is stopped or while the vehicle is left unused for a long time, the oil leaks from the mechanical oil pump and the air enters the mechanical oil pump through a gap formed in the mechanical oil pump (for example, a gap between a case and a cover), which deteriorates the rising characteristics of the hydraulic pressure produced by the mechanical oil pump during startup of the engine. An oil supply system 200 for a vehicle shown in FIG. 22 includes a mechanical oil pump 202 and an electric oil pump 204. In such oil supply system for a vehicle, when a power switch is turned on to allow the vehicle to run, only the electric oil pump 204 is actuated first to achieve a predetermined hydraulic pressure. However, the oil does not reach an inlet port of the mechanical oil pump 202 easily. Therefore, even when an engine 206 is started in response to, for example, depression of an accelerator pedal, the mechanical oil pump 202 runs idle due to presence of the air. As a result, the hydraulic pressure produced by the mechanical oil pump 202 does not exhibit appropriate rising characteristics. Therefore, a large-capacity pump needs to be used as the mechanical oil pump 204. In addition, the time at which the electric oil pump 204 is stopped in response to the actuation of the mechanical oil pump 202 is retarded. As a result, the fuel efficiency is reduced.
In an oil supply system 210 shown in FIG. 23, a check valve 212 and a check valve 214 are provided in the oil discharge passage for the mechanical oil pump 202 and the oil discharge passage for the electric oil pump 204, respectively, in order to increase the motor efficiency when each of the mechanical oil pump 202 and the electric oil pump 204 is operated by itself. In such oil supply system, if the oil leaks from the mechanical oil pump 202 and the air enters the mechanical oil pump 202 through a gap formed in the mechanical oil pump 202 when the engine 206 is stopped, for example, when the motor-power cruise mode is selected in the hybrid vehicle, the hydraulic pressure produced by the mechanical oil pump 202 does not exhibit appropriate rising characteristics due to presence of the air when the engine 206 is restarted to shift the cruise mode to the engine-power cruise mode. In addition, the air remains in the mechanical oil pump 202 because the check valve 212 is kept closed by the hydraulic pressure from the electric oil pump 204. Therefore, the mechanical oil pump 202 continues running idle, which further deteriorates the rising characteristics of the hydraulic pressure produced by the mechanical oil pump 202. Similar problems may occur in the case where only the electric oil pump 204 is actuated first when a driver turns the power switch on, and then the engine 206 is started in response to, for example, depression of the accelerator pedal.
When the hybrid vehicle is in the motor-power cruise mode, the engine 206 is stopped and a predetermined hydraulic pressure is achieved by the electric oil pump 204. As in the oil supply systems 200 and 210 in which an inlet port (strainer in FIGS. 22 and 23) 216 is shared by the mechanical oil pump 202 and the electric oil pump 204 in order to save space, the oil is sucked into the electric oil pump 204 also from the mechanical oil pump 202 side due to a vacuum pressure produced by the electric oil pump 204. Therefore, the air may enter the mechanical oil pump 202 through a gap formed in the mechanical oil pump 202. The air that has entered the mechanical oil pump 202 is eventually sucked into the electric oil pump 204 along with the oil due to a vacuum pressure produced by the electric oil pump 204, as shown in FIG. 24. At this time, air-sucking noise may be caused.